Electrochemical cells provide electrical energy that powers a host of electronic devices such as external and implantable medical devices. Among these many medical devices powered by electrochemical cells are external medical drills and implantable cardiac defibrillators. Such medical devices generally require the delivery of a significant amount of electrical power over a relatively long period of time, for example up to ten years. Thus, these devices typically require the use of electrochemical cells that are capable of providing an increased discharge capacity at an increased operating voltage. As defined herein, “discharge capacity” (Ah) is the maximum amount of electrical current that can be drawn from a cell under a specific set of conditions. The term, “specific capacity” (Ah/g) is defined herein as the maximum amount of electrical current capable of being provided on a per gram basis of cathode active material when paired with an anode in an electrochemical cell. The term, “operating voltage” is defined herein as the voltage required for the proper operation of an electrical component, electrical circuit, device or system. Thus, providing an increased discharge capacity at an increased operating voltage enables the operation of higher voltage demanding devices over longer periods of time.
Cathode chemistries such as iron disulfide (FeS2) have been developed to provide increased discharge capacities that meet the power demands of external and implantable medical devices. Iron disulfide cathode material is generally known to have a specific capacity with a lithium anode ranging from about 700 mAh/g to about 890 mAh/g, which is well suited for powering implantable medical devices over long periods of time. However, lithium electrochemical cells constructed with cathodes comprised of iron disulfide generally suffer an irreversible voltage loss on the order of about 2-3 V depending upon the electrolyte and amount of the applied load at the beginning of cell discharge. This results in the delivery of a reduced amount of operating voltage. Such a cathode chemistry is, therefore, not ideal for powering devices that require an increased operating voltage over long periods of time.
The applicants, therefore, have developed a new iron nickel disulfide cathode material formulation and cathode thereof that provides a lithium electrochemical cell having a discharge capacity that is greater than lithium cells comprising cathodes of iron disulfide. Furthermore, lithium electrochemical cells having cathodes comprised of the iron nickel disulfide cathode active material do not exhibit the initial irreversible voltage drop to the extent exhibited by comparable lithium cells comprising iron disulfide cathode active materials. Consequently, a cathode composed of the iron nickel disulfide material of the present invention when constructed within an electrochemical cell having a lithium anode is well suited for powering a variety of electrical devices that require a “high” discharge capacity and an increased operating voltage.
Thus, as will be discussed in more detail, the iron nickel disulfide cathode active material of the present invention comprises a unique chemical structure that provides a lithium electrochemical cell with electrical properties that are well suited to power a variety of electrical devices that require an increased discharge capacity with increased operating voltage capability.